[clang] [lex] Return false from __has_declspec_attribute() if not explicitly enabled

[tbaeder]

Summary

Currently, projects can check for __has_declspec_attribute() and use it accordingly, like so:

#if __has_declspec_attribute(noreturn)
__declspec(noreturn)
#endif
static void foo(void) {}

(with maybe an additional defined(__has_declspec_attribute) check around).

However, clang will by default accept this code and return true for __has_declspec_attribute(noreturn), just to error out because declspec attributes have not been enabled explicitly:

~ ❯ clang ./declspec.c
./declspec.c:3:1: error: '__declspec' attributes are not enabled; use '-fdeclspec' or '-fms-extensions' to enable support for __declspec attributes
__declspec(noreturn)
^
1 error generated.

I think it would be better if clang simply returned false for __has_declspec_attribute() checks if declspec attributes have not been enabled.

[aaron.ballman]

Good catch, especially given that we document this as only working if the target supports the attribute (https://clang.llvm.org/docs/LanguageExtensions.html#has-declspec-attribute). LGTM with a few minor nits.

clang/lib/Lex/PPMacroExpansion.cpp

1697

clang/test/Preprocessor/has_declspec_attribute.c

7

[tbaeder]

Thanks for the quick review. I've made the requested changes and added a proper commit message. I noticed I forgot that before.

[aaron.ballman]

LGTM, thank you!

[aaron.ballman]

I've commit this on your behalf in ef3800e82169c674219501d9ac09ef12b28e6359, thank you for the patch!

clang/lib/Lex/PPMacroExpansion.cpp

EvaluateFeatureLikeBuiltinMacro(OS, Tok, II, *this,
         return II ? hasAttribute(AttrSyntax::GNU, nullptr, II,
                                  getTargetInfo(), getLangOpts()) : 0;
       });
   } else if (II == Ident__has_declspec) {
     EvaluateFeatureLikeBuiltinMacro(OS, Tok, II, *this,
       [this](Token &Tok, bool &HasLexedNextToken) -> int {
         IdentifierInfo *II = ExpectFeatureIdentifierInfo(Tok, *this,
                                            diag::err_feature_check_malformed);
-        return II ? hasAttribute(AttrSyntax::Declspec, nullptr, II,
-                                 getTargetInfo(), getLangOpts()) : 0;
+        if (II) {
+          const LangOptions &LangOpts = getLangOpts();
+          return LangOpts.DeclSpecKeyword &&
+                 hasAttribute(AttrSyntax::Declspec, nullptr, II,
+                              getTargetInfo(), LangOpts);
+        }
+
+        return false;
       });
   } else if (II == Ident__has_cpp_attribute ||
              II == Ident__has_c_attribute) {
     bool IsCXX = II == Ident__has_cpp_attribute;
     EvaluateFeatureLikeBuiltinMacro(
         OS, Tok, II, *this, [&](Token &Tok, bool &HasLexedNextToken) -> int {
           IdentifierInfo *ScopeII = nullptr;
           IdentifierInfo *II = ExpectFeatureIdentifierInfo(

clang/test/Preprocessor/has_declspec_attribute.c

+// RUN: %clang_cc1 -triple i386-unknown-unknown -E -P %s -o - | FileCheck %s --check-prefix=CHECK
+// RUN: %clang_cc1 -triple i386-unknown-unknown -fdeclspec -E -P %s -o - | FileCheck %s --check-prefix=ENABLED
+// RUN: %clang_cc1 -triple i386-unknown-unknown -fms-extensions -E -P %s -o - | FileCheck %s --check-prefix=ENABLED
+
+// Check that __has_declspec_attribute() always returns 0 if declspec
+// attributes have not been explicitly enabled.
+
+#define DECLSPEC(x) x: __has_declspec_attribute(x)
+// CHECK: noreturn: 0
+// ENABLED: noreturn: 1
+DECLSPEC(noreturn)